Fastest feet in the fossil record

Fossilised trackways in Spain have revealed an exciting story: some flesh-eating, two-legged dinosaurs could move scarily fast.

In a finding published this week in Nature: Scientific Reports, Pablo Navarro-Lorbés and colleagues from the Universidad de La Rioja, Spain, analysed two fossilised trackways in La Rioja dating from the Early Cretaceous period (145 to 100.5 million years ago), one consisting of five prints, and the other of seven. The morphology of the prints, each with three toes and a narrow profile, indicates that they were made by the same species of dinosaur. Although they can’t pinpoint the species, the researchers suspect that the tracks were made by an agile, medium-sized cousin of the Tyrannosaurus rex.

The team built photogrammetric 3D models of both the trackways and the individual footprints, which they then combined to perform speed and gait analyses. The results showed that the dinosaur that created the five-print track was accelerating smoothly at speeds between 23.4 and 37.1km/h, while the maker of the seven-print track was estimated to have been running at between 31.7 and 44.6km/h. It’s among the three fastest speeds ever estimated from this group of T.rex-like dinosaurs, which are known collectively as theropods.

The emerging picture shows more than just speed – this was a predatory dinosaur that was agile as well as fast. The seven-print track shows significant abrupt speed changes, which the authors suggest indicates the dinosaur was manoeuvring as it ran.

If this smaller cousin of the T.rex was blisteringly fast, does this mean the world’s most well-known dinosaur species was even faster? Not likely, say the researchers.

“When approaching masses greater than a tonne, bipedal non-avian dinosaurs would display lower running abilities due to the higher muscular masses needed to support the forces and stresses derived from high velocities,” they write.

With weights estimated between four and seven tonnes, the T.-rex’s bulk probably restricted its top speeds.

Mid-sized predators, such as the ones that made these tracks, likely developed their intimidating speeds in response to their “double condition as the hunters of smaller prey and the prey of bigger hunters,” note the authors.

The study offers nuance to existing knowledge of theropods, giving a glimpse of some of the less tangible aspects of their biology. With no living creatures on Earth sharing a similar mode of locomotion, many questions about how these ancient animals moved through their environment have long gone unanswered.

“The study of dinosaur tracks can help answer some of these questions due to the very nature of tracks as a product of the interaction of these animals with the environment,” write the authors.